Today's cellular phones and other wireless terminals often support two or more frequency bands (or frequency band pairs) and may also support two or more distinct cellular system standards. This is particularly true for mobile phones designed for international use, since frequency spectrum allocation remains un-harmonized throughout the world. This situation is exacerbated by the roll-out of new services on newly allocated frequency bands—mobile terminals designed for the new services must typically continue to support existing services on their corresponding frequency bands.
Support for multiple frequency bands typically requires duplication of several radio frequency (RF) components, such as filters. These RF filters are often among the bulkiest components in the mobile phone. In some cases, the performance requirements for RF filters are also very demanding, which in turn may increase the size of the filters, their cost, or both.
One approach to supporting several frequency bands in a mobile terminal is to allow only half-duplex operation in some of the bands. Half-duplex operation means that the mobile terminal does not support simultaneous transmission and reception. A key benefit of half-duplex operation is the easing of filtering requirements. In particular, a (typically large) duplex filter, with stringent requirements to prevent transmitter signals from desensitizing the receiver, is no longer required. Instead, a transmit/receive (T/R) switch is typically used, connecting a device antenna to the receiver in one state and to the transmitter in another. T/R switches are typically smaller and less expensive than a duplex filter.
Dispensing with the duplex filter has other benefits as well. For instance, a duplex filter generally has significant loss (signal attenuation), which decreases receiver sensitivity and requires the transmitter power amplifier output to be increased. Thus, half-duplex operation may provide improved receiver performance as well as improved power consumption, especially when the mobile terminal is operating at a high output power level.
However, switching between transmit and receive modes in a half-duplex transceiver requires a finite switching time, during which time the typical half-duplex transceiver can neither transmit nor receive. In a system where different frequencies are used for transmit and receive operations (a frequency-division duplex, or FDD, system), this switching time may include the time necessary to re-tune one or more local oscillators from a receive frequency to a transmit frequency. In many systems, this time is accounted for with a “guard time”—the system is designed around this gap in performance. Although the exact switching time for a given transceiver depends on the transceiver design details, this switching time may, in a high-bandwidth and/or high data-rate system, extend over several data symbols. Thus, the corresponding guard times preclude the most efficient possible use of the frequency spectrum by the half-duplex transceiver.
In the Long-Term Evolution (LTE) system currently under development by the 3rd-Generation Partnership Project (3GPP), mobile terminals may support either half-duplex or full-duplex operation. (Note that the LTE specifications provide for frequency-division duplexing, or FDD, as well as time-division duplexing, or TDD, systems. However, most systems are expected to be FDD.) As a result, a mixture of half- and full-duplex mobile terminals may be present in a given cell at any given time; the system design must accommodate this mixture.
One proposed approach for handling half-duplex terminals in LTE systems is to create an artificial guard time between receive and transmit modes by simply allowing the mobile terminal to ignore one or more of the last symbols in a sub-frame immediately preceding a mode change. In other words, the mobile terminal may begin switching slightly before the current sub-frame is completed, so that the terminal is ready to commence operation in the new mode at the beginning of the next sub-frame. An advantage of this technique is that it simplifies network design, since data is interleaved and transmitted in the same way for both half-duplex and full-duplex terminals. Complete loss of data is prevented by error correction (e.g., convolutional coding) and error detection/re-transmission schemes (e.g., hybrid ARQ). However, this solution will result in a degradation in throughput, perhaps similar to what might result from puncturing or increasing a redundancy coding rate. Depending on the coding rates used, this degradation could be significant. Thus, this previously proposed solution will degrade a half-duplex terminal's throughput as well as the overall capacity of the system.